Exosomes are believed to contain important signaling molecules that may provide the source of trophic factors responsible for some regenerative benefits seen in cell replacement therapy. As such they would provide an alternative to some cell based therapies that would be easier to manufacture on a large scale and potentially safer to administer to a subject in need of cell therapy. In particular, the risk associated with transmission of infectious agents such as viruses may be lower compared to transplanting whole cells. Moreover, the risk of immune rejection of the exosomes relative to transplanted cells may also be lower. Accordingly, exosomes may provide an attractive alternative or adjunct to cell based therapies and cell based regenerative medicine.
Exosomes are 30 to 120 nm vesicles secreted by a wide range of mammalian cell types. Keller et al. (2006) Immunol Lett. 107(2):102; Camussi et al. (2010) Kidney International 78:838. The vesicles are enclosed by a lipid bilayer and are larger than LDL which has a size of 22 nm, but smaller than a red blood cell, which is 6000 to 8000 nm in diameter and has a thickness of 2000 nm Keller et al. (2006) Immunol Lett. 107(2):102.
Exosomes are found both in cells growing in vitro as well as in vivo. They can be isolated from tissue culture media as well as bodily fluids such as plasma, urine, milk and cerebrospinal fluid. George et al. (1982) Blood 60:834; Martinez et al. (2005) Am J Physiol Health Cir Physiol 288:H1004. Exosomes originate from the endosomal membrane compartment. They are stored in intraluminal vesicles within multivesicular bodies of the late endosome. Multivesicular bodies are derived from the early endosome compartment and contain within them smaller vesicular bodies that include exosomes. Exosomes are released from the cell when multivesicular bodies fuse with the plasma membrane. Methods of isolating exosomes from cells has been described, see e.g. US Patent Application Publication No. 20120093885
Exosomes contain a variety of molecules including proteins, lipids and nucleic acids such as DNA, mRNA and miRNA. Their contents are believed to play a part in cell to cell communication involving the release of the exosome from one cell and the binding/fusion of the exosome with a second cell, wherein the contents of the exosomal compartment are released within the second cell.
It has been reported that exosomes derived from endothelial progenitor cells may act as vehicle for mRNA transport among cells. These exosomes were shown to incorporate into normal endothelial cells by interacting with the α4β1 integrin. Once incorporated into the endothelial cells, the exosomes stimulated an angiogenic program. Deregibus et al. (2007) Blood 110:2440. Similar results were obtained in vivo using severe combined immunodeficient mice. Exosome stimulated endothelial cells implanted subcutaneously in Matrigel (a murine sarcoma extract) organized into a patent vessel network connected with the murine vasculature. Deregibus, supra. Bruno et al. (2009) J Am Soc Nephrol 20:1053; Herrera et al. (2010) J Cell Mol Med 14:1605.
Of the various molecular cargo of exosomes, miRNAs have recently attracted a lot of attention due to their regulatory roles in gene expression. MiRNAs are small, non-coding regulatory RNAs that can have a wide range of effects on multiple RNA targets, thus having the potential to have greater phenotypic influence than coding RNAs. MiRNA profiles of exosomes often differ from those of the parent cells. Profiling studies have demonstrated that miRNAs are not randomly incorporated into exosomes but rather a subset of miRNAs is preferentially packaged into exosomes, suggesting an active sorting mechanism of exosomal miRNAs. Guduric-Fuchs et al. (2014) Nucleic Acid Res. 42:9195; Ohshima et al. (2010) PloS One 5(10):e13247.
Because exosomes contain a variety of molecules, many believed to play an important role in cell signaling, exosomes would prove useful in research and industry and would have applications as therapeutics, diagnostics and in screening assays. Frequently, however, the availability of reproducible, essentially identical populations of exosomes is limited by the fact that most sources of exosomes are cells that senesce and thus have limited replicative capacity. Accordingly, there is a need for exosomes that are derived from a clonal source that has an extended replicative capacity that is greater than most adult or fetal derived cells. The invention described infra meets this need and as well as other needs in the field.